Causes of Bleeding in the First Trimester

A review of the various causes of bleeding during the first trimester of pregnancy.

Most couples expect to get pregnant at some point, and when they do for the first time it suddenly dawns on them what a gamble pregnancy actually is. One of the most frightening things is to experience bleeding in the first part of the pregnancy. First trimester bleeding is any bleeding noted during the first 12 weeks of pregnancy, and it is one of the most common symptoms to send a woman to her obstetrician. And rightly so, because until a non-threatening cause is identified, all first trimester bleeding is labeled “threatened miscarriage,” or “threatened AB.”

Possible causes of bleeding in the first trimester unrelated to miscarriage:


Cervicitis is a condition in which the delicate cells at the mouth of the womb (or the cervix) bleed due to the mechanical action of intercourse, the alteration of acidity in the vagina (pH), or the effects of infections on these cells. There are two causes of Cervicitis:

  1. Ectopy (ectropion). With the hormonal changes of pregnancy, the fragile internal cells peek out a bit onto the external portion of the cervix, which is a harsher environment for them. Normally nestled deeply away from sexual activity and the acidity of the vagina, they can now be battered both chemically and mechanically. They’re easily damaged, causing bleeding. Of course, we’re not talking about a whole lot of bleeding – merely what is perceived as spotting. It must also be noted that these cells don’t normally bleed with sex –there is usually a predisposing condition, such as cervicitis. When these internal cervical cells are brought to a more external position, this is called ectopy.
  2. Infection. Cervicitis is inflammation due to infection. Yeast is the most common culprit, and a simple prescription or even over-the-counter cream can end this concern quickly. Other infections are more worrisome. Sexually transmitted diseases (STDs), such as gonorrhea, chlamydia, trichomonas, and Gardnerella can do the same, so a microscopic evaluation is the best approach rather than just assuming it’s yeast. Some infections may be silent for years, meaning that even though there is no question of fidelity in a couple, there may have been an infection long before they even met each other; because of this, cultures for STDs have become standard in all pregnancies.

Cervical Polyps

Harmless small polyps can also cause bleeding. These are overgrowths of benign tissue, probably owing their existence to estrogen levels that made them grow. Most often they can be gently and painlessly twisted off during a physical exam. If not, they’re usually destroyed by the very act of delivering the baby. It’s worth getting them off at a checkup so that your doctor won’t need to worry and feel obligated to force you into a lot of extra tests you don’t need every time there’s spotting.

Subchorionic Hemorrhage

This sounds disastrous, but it usually represents a small clot that causes bleeding and then dissolves away harmlessly. Rarely, the clot dissects between the placenta and the attachment to mother and causes a miscarriage.

Decidual Tissue

Sometimes a small piece of tissue becomes loose and disintegrates through some unknown cause, causing spotting. It’s usually a hormonally stimulated collection of menstrual-like tissue that can often be confused with a miscarriage. If it’s just tissue debris, it can mean nothing. If it’s actual tissue of the pregnancy (fetal or placental), then there should be serious concern, because now this “threatened miscarriage” is re-labeled as “incomplete miscarriage.” No one knows why such a phenomenon occurs, but it is harmless. It’s the passage of tissue for sure, so it’s very disturbing until the pathology report can ease everyone’s mind. Strangely enough, the pseudopregnancy of birth control pills can cause this type of tissue shedding, which is really upsetting to a woman who thought (correctly) that she had been using her contraceptive properly. The stabilization of the uterine lining depends on estrogen and progesterone. It’s possible that there are lags in the amounts of hormones so that lining not involved with implantation loses its grip and sheds. In my practice, such shedding of only decidual tissue has had no impact on whether a pregnancy will miscarry. The tissue is termed “decidualized,” because of the pregnancy-like effect on it at the hands of estrogen.

Implantation Bleeding

In the past it’s been thought that an egg eroding into the uterine lining would cause bleeding at the time because of a burrowing effect. It’s doubtful whether there’s any bleeding when this happens, and if so, it’s too small an amount to notice. The myth persists because there are bleeding episodes in which no cause is ever identified and in which the pregnancy goes on successfully to term. Such a mystery that starts off so menacingly but ends so well begs for an explanation that must include a natural process. Implantation makes sense under these criteria but can’t be proven.

Periods During Pregnancy

Many women ask me about having regular periods during their pregnancies. They’re concerned because Grandma or a cousin had periods every month during pregnancy, and could that happen during this pregnancy, making the last menstrual period (whichever one that was) screw up the due date? They swear that these periods during pregnancy are true – that they really happened. It’s false, all the swearing notwithstanding. Shedding one’s layer of menstrual tissue is not compatible with life. The closest thing we have to this is shedding of decidual tissue (as described above). When Grandma swears that it happened, it’s certainly the polite thing to listen with an open mind – just be sure to slam it shut by thinking about what’s really going on in pregnancy. The cycling of hormones stops because a pregnancy causes the hormone levels to remain high. This is necessary for pregnancy to continue. There are no falls in the hormones, which is what causes a period, except right before labor. Most likely, Grandma experienced a subchorionic hemorrhage (as described above), bleeding intermittently, misinterpreted as cyclic. That would explain her first trimester “periods.” Just being Grandma might explain the rest of her pregnancy’s periods.

Although the above instances describe the causes of bleeding that do not indicate miscarriage, miscarriage should still be ruled out if you have any bleeding at all. And when one considers that the cramping of a threatened miscarriage can feel exactly like the growing pains of a normal uterus, it is fortunate that there are other tools to give you peace of mind.

Blood tests can prove that the pregnancy hormone is increasing as expected and confirms a healthy pregnancy; ultrasound can demonstrate the physical well-being of a growing baby by showing a healthy heart rate or by ruling out an ectopic (tubal) pregnancy.

It’s true that miscarriage is a fact of life as we know it, and usually it’s due to some sort of doomed genetic mismatch; but although most miscarriages begin with first trimester bleeding, first trimester bleeding isn’t always indicative of a miscarriage. Doctors always approach first trimester bleeding as a possible miscarriage until a cause can be determined. Usually it has a good outcome. So, although it’s understandable how first trimester bleeding can cause a lot of anxiety and worry, your doctor can usually find something unrelated to the pregnancy – and treatable – to blame it on.

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